NAME

       attimer — i8254 Programmable Interval Timer (AT Timer) driver

SYNOPSIS

       This driver is a mandatory part of x86 kernels.

       The following tunables are settable from the loader(8):

       hint.attimer.X.clock
       controls  support  for  the event timer functionality.  Setting this value to 0 disables it.  The default
       value is 1.

       hint.attimer.X.timecounter
       controls support for the time counter functionality.  Setting this value to 0 disables it.   The  default
       value is 1.

       hw.i8254.freq
       allows  overriding  the  default counter frequency.  The same value is also available at run-time via the
       machdep.i8254_freq sysctl.

DESCRIPTION

       This driver uses i8254 Programmable Interval Timer (AT Timer) hardware to  supply  the  kernel  with  one
       timecounter  and  one  event  timer,  and  to generate sound tones for the system speaker.  This hardware
       includes three channels.  Each channel includes a 16 bit counter which decreases with a known,  platform-
       dependent  frequency.   Counters can operate in several different modes, including periodic and one-shot.
       The output of each channel has platform-defined wiring: one channel is wired to the interrupt  controller
       and may be used as event timer, one channel is wired to the speaker and used to generate sound tones, and
       one timer is reserved for platform purposes.

       The  attimer  driver  uses  a  single  hardware  channel  to  provide  both  time counter and event timer
       functionality.  To make this possible, the respective counter must be running in  periodic  mode.   As  a
       result, the one-shot event timer mode is supported only when time counter functionality is disabled.

       The event timer provided by the driver is irrelevant to CPU power states.

SEE ALSO

       apic(4), atrtc(4), eventtimers(4), hpet(4), timecounters(4)

Debian                                            May 26, 2014                                        ATTIMER(4)